As is known, in several technological processes for converting plastic materials, solid powders, commonly known as mineral fillers, are used which must be mixed with the components in order to obtain the final product.
Typically, solid powders are used to give the final manufactured article being produced better physical and mechanical characteristics, and in other cases the solid powders merely have the function of an inert product in order to reduce the cost of the final manufactured article and to be able to recycle plastic material waste by means of a mechanical recycling process.
The application of solid powders is commonly used in continuous or discontinuous processes, also in the production of expanded polyurethanes, in which solid powders are introduced which are constituted both by mineral fillers and by finely pulverized production waste.
Typically, the solid powders must be dosed and premixed with a first process liquid, which is normally polyol, so-called reagent liquid, and this mixing is performed by using a mixing tank, technically known as “batch system”, or by using a dynamic mixer directly in-line, with a method commonly known as “in-line system”.
The mixture that is formed, in both systems, is conveyed to the intake of a dosage pump, whose line leads to the mixing head, where the line of the other reagent liquid for forming the polyurethane polymer, more precisely isocyanate, is provided.
The mixing heads that are used can be low- or high-pressure.
The systems for dosing solid powders currently used suffer considerable problems as regards dosage precision which, by being typically provided by means of a single- or double-screw type, inevitably generates dosage variations, since the powders are compressible and this error becomes considerably greater if the powder has to be introduced in a pressurized circuit.
Prior solutions, disclosed, for example in U.S. Pat. Nos. 5,152,943, 5,547,276 and 5,332,309, provide devices which use compression dosage screws so as to dose the solid powders in the liquid with a certain pressure value.
As is known, in these embodiments, a variation of the density of the solid powder entails an anomaly in the dosage, and therefore a system for feedback of the signal related to the pressure developed in the premixer chamber is used, which increases or decreases the rotation rate of the dosage screw by comparing it to a set pressure value, accordingly modifying the degree of compression of the solid powder in order to keep the dosage constant over time.
It is evident that maintaining the dosage of the solid powders at a constant value encounters considerable difficulties, which derive from the slowness of response in the system and from the fact that, in the solutions of the background art, the pressure variation in practice is entrusted directly to the variation of the rotation rate of the screw, with the inevitable associated inaccuracies.